Not Applicable.
1. Field of Invention
This invention relates to an automatic pole-zero adjustment circuit for an ionizing radiation spectroscopy system. Specifically, this invention relates to a circuit for an ionizing radiation spectroscopy system which automatically adjusts the pole-zero based upon the time constant of the output of the preamplifier.
2. Description of the Related Art
Semiconductor radiation detectors are commonly used to measure the characteristics of ionizing radiation. Typical examples of such radiation are X-rays, gamma rays and charged particles such as alpha particles. The detector measures the energy of single photons or particles. Often it is the case that the characteristic of interest is the energy spectrum. The spectrum is a chart or histogram of the number of events occurring as a function of energy. Discrete energies show up as peaks in the spectrum. If the detector system has good energy resolution, then the peaks will be narrow and closely adjacent energies can be distinguished.
When the detector absorbs the energy of the photon or the charged particle, free electronic charge is produced inside the detector. The charge produced is accurately proportional to the energy absorbed. Typically, the signal current produced by the collection of the free charge is integrated on the feedback capacitor of a preamplifier. The output voltage of the preamplifier is thus equal to the charge divided by the feedback capacitor and is used as a measure of the energy of the ionizing radiation. If the voltage is measured for many events and charted as the number of events versus voltage, then the characteristic energy spectrum of the radiation is obtained.
Unless some means is provided to remove charge from the feedback capacitor, the output voltage will soon reach limits imposed by the circuit and the system will cease to function. The most common solution to this problem is to place a feedback resistor in parallel with the capacitor. The preamplifier output then consists of voltage steps proportional to the energy of individual energy absorption events, separated by exponential decays having a time constant equal to the product of the feedback resistance and the feedback capacitance. In order to maintain the excellent signal-to-noise ratio inherent in the detector, the resistance must be large. Typical time constants are in the millisecond range.
For optimum energy resolution and high data rates, the pulse from the preamplifier must be filtered and amplified. The filter amplifier changes the shape of the output to a pulse with a longer rise time and shorter decay time than the preamplifier pulse. The maximum amplitude of the filtered pulse is still proportional to the absorbed energy. Measurement of the pulse amplitude relative to the baseline between pulses gives the best estimate of the energy. Clearly, the baseline must be constant and stable to at least the same accuracy as the energy measurement.
The first stage of the filter amplifier is typically a high-pass filter. The high-pass filter removes low frequency noise and shortens the width of the pulse. If the exponentially decaying preamplifier pulse is passed through a simple capacitor/resistor high-pass filter, then the resulting output is an exponential decay having the time constant of the high-pass filter followed by an undershoot having the original preamplifier decay time constant. This undershoot is very undesirable. The amplitude of subsequent pulses is reduced by the amount of the residual undershoot, when it occurs, making the results inaccurate. Unless the data rates are kept unreasonably low, the result will be distortion of the individual peaks in the energy spectrum.
A technique to eliminate the undershoot was disclosed by Nowlin, et al., in an article entitled xe2x80x9cElimination of Undesirable Undershoot in the Operation and Testing of Nuclear Pulse Amplifiers,xe2x80x9d Rev. Sci. Instr., Vol. 36, No. 12, Dec. 1965, pp 1830-1839, the contents of which are fully incorporated here by reference. In Laplace transform terms, the preamplifier decay time constant, or pole, is canceled by a zero in the high-pass filter that coincides with the pole in the complex frequency or xe2x80x9csxe2x80x9d plane. Nowlin""s method, commonly known as pole-zero cancellation, performs well, but requires added equipment and a skilled operator to correctly adjust. The pole-zero cancellation is considered complete when the operator, using an oscilloscope or other indicator, observes minimum under/overshoot of the pulses.
If the entire system of detector, preamplifier, and filter amplifier were assembled by the same manufacturer at the same time, the pole-zero adjustment would be performed at the manufacturing site, using optimum equipment, and thus would cause no problems for the end user. Often, however, the detector and preamplifier are assembled as a unit and the filter amplifier and other electronic components of the spectroscopy system are assembled separately, perhaps by a different manufacturer. The user may wish to upgrade either the detector or the electronics at some later date. Because of these factors, it is necessary that the pole-zero cancellation be possible for an arbitrary combination of a detector/preamplifier unit and a filter amplifier.
Typically, the preamplifier contains a high-pass filter to replace the original long time constant pole with some shorter value pole, for example, 50 microseconds. The pole-zero cancellation required at this stage is done by the manufacturer and normally never changed. The typical value used is longer than that required for optimal performance so the filter amplifier must still contain a high-pass filter and a pole-zero cancellation circuit to cancel the preamplifier""s output pole.
Britton, et al., described a method mathematically equivalent to the Nowlin method, but performed automatically, in U.S. Pat. No. 4,866,400, fully incorporated herein by reference. Britton, et al., teach monitoring the under/overshoot of the pulses from the analog filter amplifier to control the pole-zero cancellation network in the high-pass filter. The pole-zero cancellation is considered complete when minimum under/overshoot is observed.
Bingham, et al., in U.S. Pat. No. 5,872,363, fully incorporated herein by reference, describe a method similar to the Britton, et al., method except applicable to the case wherein some or all of the high-pass and subsequent filtering are done digitally. After converting the signal to digital form, using an analog-to-digital converter, the pulses are filtered using digital signal processing techniques. Incorrect pole-zero cancellation causes under/overshoot of the digital pulses resulting in spectrum distortion similar to that in the analog system. Bingham, et al., teach monitoring the under/overshoot of the digital pulses to control the pole-zero cancellation in either an analog high-pass or all digital filter. The pole-zero cancellation is considered complete when minimum under/overshoot is observed.
Another method is to monitor the shape of the peaks in the spectrum to control the pole-zero cancellation in either an analog high-pass filter or a digital filter. The pole-zero cancellation is considered complete when minimum spectral peak distortion caused by under/overshoot is observed.
All of the above techniques can be described as closed-loop or feedback controllers. The output of the system, either the under/overshoot of the filtered pulses or the resulting spectral peak distortion, is used to generate a suitable control signal to adjust the pole-zero adjustment circuit. The pole-zero adjustment process is repeated until an acceptable result is achieved. After adjustment, the parameters of the pole-zero adjustment circuit are left constant for subsequent data collection.
Pole-zero adjustment can also be accomplished by an open-loop system without using feedback. Open-loop systems are typically simple to design and inexpensive but not as accurate as closed-loop systems. If the preamplifier decay time constant (the pole) of the input to the pole-zero cancellation circuit is accurately known, then the pole-zero adjustment is simply performed by setting the zero of the pole-zero circuit to coincide with the input pole. If the pole-zero cancellation circuit is programmable, then the adjustment can be done easily in a computer-controlled system, requiring little or no skill on the part of the operator. If the pole-zero cancellation network is precise and stable, then the results can approach that of the closed loop systems.
It is an object of this invention to provide open-loop automatic pole-zero cancellation circuits and methods that are simpler than the closed-loop methods of the prior art but provide essentially equal performance. It is a further object to provide open-loop automatic pole-zero cancellation circuits and methods such that automatic pole-zero cancellation for arbitrary combinations of detector/preamplifier units and filter amplifiers can be used with little or no operator input.
An automatic pole-zero (APZ) adjustment circuit for an ionizing radiation spectroscopy system is provided. The detected radiation emissions are fed into a preamplifier with a conventional parallel RC feedback circuit and passed to a high pass filter. The high pass filter improves the signal-to-noise ratio but the exponentially decaying input of the high pass filter results in undershoot. Undershoot is canceled by adding a correction signal generated by a pole-zero adjustment network. The correction signal is selected to algebraically cancel the undershoot when summed with the output signal of the high pass filter.
The high pass filter output and the pole-zero adjustment signal are summed and passed to the input of a sampling analog to-digital converter (ADC). Typically, these signals are currents and the summing is performed by a summing amplifier. The sampling ADC samples and converts the combined signals to a digital signal. The digital signal passes through a digital shaping filter that improves the precision of the energy measurement by removing higher, frequencies thereby improving the signal-to-noise ratio and minimizing the effects due to variable rise times and baseline errors. The digital shaping filter results in a pulse which has a longer rise time but still represents the energy of the detected emission.
A pulse amplitude sampling circuit samples the peak amplitude of each pulse output from the digital shaping filter. A histogram containing the number of pulses at each different voltage level is recorded by an amplitude histogram circuit. The histogram, which displays one or more peaks identifying the nature of the radiant emission detected, may be viewed in a display.
A sampling ADC samples the preamplifier output under the control of a processing device, typically a microprocessor. The processing device uses averaging and fitting techniques to calculate the exact preamplifier decay time constant. Using the calculated preamplifier decay time constant and the known characteristics of the pole-zero adjustment network, the processing device derives the correct pole-zero adjustment network control input. The correction signal from the pole-zero adjustment network is combined with the output of the high-pass filter to produce the pole-zero canceled signal. A balanced pole-zero network results in a peak having minimum distortion as evidenced by a peak having a minimum width and a near Gaussian distribution, while an unbalanced pole-zero network results in an asymmetric bell-shaped distribution showing low side distortion (undershoot) or high side distortion (overshoot).
In an alternate embodiment of the automatic pole-zero circuit, the high pass filter, the pole-zero adjustment network, and the summing amplifier are eliminated and the compensation is accomplished directly in a programmable digital shaping filter which transforms the exponential pulse shape by applying a digital filter using the calculated time constant of the preamplifier to correct the signal.